Waveguide impedance matching system



M. c. CROWLEY-MILLING 2,786,98 0

WAVEGUIDE IMPEDANCE MATCHING SYSTEM Filed July 17, 1952 Fig.1.

Inventor- Michael OCrowle y-Milling His Attorney.

WAVEGUIDE MPEDANCE MATCHING SYSTEM Michael C. Crowley-Milling, Colwyn Bay, Wales, assignor to Metropolitanickers Electrical Company Limited, London, England, a British company Application July 17, 1952, Serial No. 299,482

Claims priority, application Great Britain July 24, 1951 12 Claims. (Cl. 3339) This invention relates to radio and radar aerial systerns. More particularly, the invention is concerned with centimeter wave aerials or antennas of the kind comprisng a reflecting surface or refracting medium so shaped that during transmission the energy from a primary feed system is concentrated thereby into a desired beam or pattern.

In most usual forms of aerial, the reflecting or re fracting medium reflects back part of the energy into the primary feed or transmission system, which normally comprises a waveguide or coaxial transmission line, setring up standing waves in the transmission system. Th'm is undesirable particularly, if, as in most radio systems, a magnetron is used as the source of transmitted power, inasmuch as the power output and frequency stability of a magnetron are very sensitive to variations of load impedance.

The phase of the reflected power at the primary feed changes rapidly with changes in frequency due to the length of path from the primary feed to the reflector or refractor and back; it may be as great as 50 wavelengths. Thus, a change of 1% in frequency may make a change of 180 in the phase of the reflected wave.

l-leretofore, it has been proposed to fit an auxiliary reflecting device such as an iris or probe to the primary feed of such an aerial in such a position as to cause an additional reflection which, at the desired operating fre' quency, is equal and opposite in phase to that caused by the main reflector or refractor. It will, however, be obvious that inasmuch as the phase of the latter reflection changes so rapidly with changes of frequency, such a device is effective only over a very narrow band.

It has also been proposed to place an auxiliary reflecting device near the main reflecting or retracting surface so that the distance between the two reflections, and thus the phase change with change of frequency, is smaller. Such a device may be a small plate placed a quarter of a wave length in front of the main reflecting surface of a parabolic dish opposite to the primary feed, or it may be provided by dividing the lens aerial into two or more sections displaced axially by quarter wavelengths.

While these methods are somewhat successful in increasing the band over which such an aerial can be used, they introduce other disadvantages. Thus, some change is nearly always caused in the polar diagram of radiation, usually in the form of an increase of unwanted side lobes.

It is, therefore, a main object of this invention to provide a novel and improved arrangement for reducing or minimizing the reflections above referred to.

In carrying out my invention in one form, the feed system between the transmitter and the aerial is coupled to an auxiliary transmission line so designed that energy fed to it is reflected back to the transmitting apparatus in substantially opposite phase to reflections arising from the aerial, whereby cancellation of the latter reflections is accomplished.

States Patent F Patented Mar. 26, 1957 "ice In cases in which the transmitting apparatus is coupled to the aerial through a waveguide, the auxiliary line may be an auxiliary waveguide, one end of which is coupled to the main waveguide and whose electrical length at the working frequency or middle of the band is either slightly longer or shorter than the electrical distance between the coupling point and the reflector or refractor by an amount such that the respective reflections are substantially in opposite phase.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which .Fig. l is a plan view of a horizontally polarized aerial system embodying the invention; and Fig. 2 is a side view with parts broken away.

In Fig. 1 of the drawing, an antenna or aerial feed system is shown comprising a parabolic reflecting sur- :face 1 which is bounded by top and bottom plates 2, 2

and is fed from the flared end 3 of a conventional waveguide 4 serving as the transmission line between a transmitter (not shown) and the aerial.

As shown in Fig. 2, an auxiliary waveguide section 5 having the same propagation characteristics as the waveguide 4 is coupled to the waveguide 4 at a point near the transmitter by means of a directional coupler 6 so that a small proportion of the transmitted power is diverted into this waveguide.

The energy-coupling arrangement of the directional coupler 6 can be of any suitable conventional type, here shown as of the two-hole coupler variety in which the socalled main and auxiliary waveguides 4 and 5, respectively, have a common wall 11 in which a pair of spaced apertures 13, 15 are formed having suitable dimensions to provide a desired coupling factor and spaced with the centers thereof substantially a quarter wavelength at the working frequency or center of the band.

As is well known, a predetermined fractional part of the forward-travelling waves in the main waveguide 4 is coupled into the auxiliary guide 5 through the apertures 13 and 15, the waves set up by the apertures in the downward direction in guide 5 being out of phase and cancelling, the waves directed upwardly in guide 5 being in the same phase and reinforcing.

One end of the waveguide 5 is closed by means of a short-circuiting plunger 7 so that the reinforced energy in the Waveguide 5 is reflected back to the directional coupler 6 where most of it is absorbed in a matched termination 8 provided for that purpose. However, a portion of the energy reflected from the plunger 7 is coupled through the directional coupler apertures 13, 15 back into the main guide in the direction of the transmitter (not shown). If the electrical length of the path from the coupler 6 to the short-circuiting plunger 7 is made a quarter of a wavelength longer or shorter than the electrical line from the coupler 6 to the central surface of the main reflector 1, and if the coupling factor is chosen correctly, the wave reflected from the plunger 7 back through the coupler 6 to the transmitter will cancel the wave reflected from the main reflector.

Since the lengths of the paths travelled by these two waves are nearly the same, changes of frequency will cause nearly the same phase changes in the two waves at the coupler 6 and so they will very nearly cancel. Thus, the aerial will be effective over a relatively wide 'band of frequencies.

As an alternative arrangement, the waveguide 5 can be made three-quarters of a wavelength shorter than the path to the aerial reflector and this can be advantageous due to the slight difference of dispersion between the two paths.

A further feature of the invention suitable in cases where it is diflicult to make the directional coupler with exactly the correct coupling factor consists in designing the directional coupler to pass too great a power from the transmission line and then inserting a well-matched lo'ssy attenuator -17 of any suitable convention type and disposed in the auxiliary guide so that the amplitude of the reflected wave fed back from the auxiliary guide can be controlled. adjustable, the phase can be adjusted as required.

Although I have described my novel auxiliary reflecting-device as employed in an antenna feed system, it will be understood that this application of the invention is merely illustrative of various uses to which it may be put. Other applications will be apparent to those skilled in the art. For example, in instances where a transition is provided between stationary and rotating Wave-transmissio'n means, difficulty in providing smooth electrical transitions commonly results in reflection-producing discontinuities. The undesired efiects of such discontinuities canbe minimized by the wave-cancellation apparatus of my invention. in general, the auxiliary reflecting device can be used to cancel reflections in any transmission line wherein undesired reflections are produced by'sources of reflection inaccessible for impedance matching.

While I have shown and described specific embodiments of my invent-ion, I do not desire my invention to be limited to the particular form shown and described and I intend by the appended claims to cover all modifications within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. The combination comprising a main transmission line having an input end and an output end and an impedance discontinuity at a point toward its output end, a directional coupler coupling to a point on said main line intermediate its input and said impedance discontinuity, a compensating transmission line, the electrical length of said compensating line differing from the electrical length. of said main line between the coupling point and the impedance discontinuity by substantially one-quarter wavelength of the median frequency 0'? the band of frequencies to be transmitted through sa l main line, said coupler coupling a first end of said compensating line to said intermediate point on said main hne, for the directtonof propagation from the input to the output of said main line, the other end of said compensating line being terminated by an electrically reflecting termination, said reflecting termination being adjustable along said compensating line, and an energy-absorbing, impedancematching termination, said directional coupler coupling said last-mentioned termination to said main and compensating lines for the direction of propagation from said output to said input ends of said main line and from said terminated end to said first end of said compensating line, whereby the standing wave at the input end of said main line resulting from reflection of energy by said impedance discontinuity can be reduced by adjustment of the said reflecting termination along said compensating line to produce an opposing or compensating reflection at the inputend of said main line.

2. The combination of claim l, in which the coupling coefiicient of the directional coupler c is equal to the square root of the expression Where a is the absolute value of the ratio of the reflected electric field to the incident electric field.

3. The combination of claim 1, in which the coupling If the short-circuiting plunger '7 is made.

coelficient of the directional coupler c is greater than the square root of the expression where a is the absolute value of the ratio of the reflected electric field to the incident electric field, and an adjustable attenuator included in the compensating transmission line.

4. The combination of claim 1, in which said transmission lines and said directional coupler are of the single hollow conductor type of wave-guide structures.

5. The combination of claim 1, in which said transmission lines and said directional coupler are coaxial line structures.

6. In combination, a main single hollow conductor type wave-guide transmission line having an input end and an output end and an impedance discontinuity near said output end, a single hollow conductor type waveguide directional coupler coupled to said main wave guide at a position intermediate said input end and said discontinuity, a compensating single hollow conductor type wave-guide transmission line one end of which is closed and the other end of which is reflectively terminated and coupled by means of said coupler to said input end of said main wave guide, the electrical length of said compensating line dilfering from the electrical length of said main line between said position of said coupler and said discontinuity by a quarter Wavelength of the median frequency of the frequency band to be transmitted through said main transmission line, and an energy-absorbing termination connected by meansof said directional coupler to said compensating line and the output end of said main transmission line, the transmission properties of said directional coupler and said compensating line being proportioned to produce a reflected energy level on said main line substantially equal to the reflected energy level produced on said main line from said impedance discontinuity.

7. The combination of claim 6, in'which the coupling coefiicient of the directional coupler c is equal to the square root of the expression Where a is the absolute value of the ratio of the reflected electric field to the incident electric field.

8. The combination of claim 6, in which the length of the compensating line is adjustable.

9. The combination of claim 3, in which the transmission lines and associated devices are of the single hollow conductor type of wave-guide structures.

10. The combination of claim 3, in which the transmission lines and associated devices are coaxial line structures.

'11. High frequency apparatus, comprising a main section of transmission line adapted to propagate high frequency wave energy of characteristic wave length from a source at a first location to a utilization device at another location, said main section of said transmission line being traversed in a direction from said utilization device location to said source location by reflected energy originating in the region of said utilization device, an auxiliary transmission line provided with a reflecting termination at one end and a substantially reflectionless termination at the other end, and a directional coupler operatively connected between said main transmission line and said auxiliary transmission line in a region between said source location and said utilization device on said main transmission line and at a region intermediate the length of said auxiliary transmission line spaced'from the reflecting termination of said auxiliary transmission line a distance which is substantially one quarter 'of'said wave length different from the distance between said directional coupler and said utilization location on said main transmission line, the sense of operation of said directional coupler being arranged to effectively transfer a portirn of the energy traveling along said main transmission line from said source location to said utilization device location into the portion of said auxiliary transmission line provided with said reflecting termination directed toward said reflecting termination, and the transmission characteristics of said directional coupler and said auxiliary transmission line being further proportioned to provide on said main transmission line a level of reflected energy from said aux iliary transmission line substantially equal to the level of reflected energy produced on said main transmission line from the region of said utilization device.

12. In combination, a source of high frequency energy having a characteristic Wave length, a utilization device, a main section of transmission line connected between said source and said utilization device and propagating energy from said source to said utilization device, said utilization device being adapted normally to set up un desired reflections in said main line section, an auxiliary transmission line provided with a reflecting termination at one end and a substantially reflection-less termination at the other end, and a directional coupler operatively connected between said main transmission line and said auxiliary transmission line in an intermediately disposed region of said main transmission line and a region of said auxiliary transmission line situated at a distance from the reflecting termination of said auxiliary line differing by one quarter of said wave length measured along said auxiliary transmission line from the distance between the region of said directional coupler and said utilization device measured along said main transmission line, the sense of operation of said directional coupler being arranged to deliver energy traveling from said source into said auxiliary transmission line directed toward said reflecting termination, and the transmission characteristics of said directional coupler and said auxiliary transmission line being further proportioned to provide on said main transmission line a level of reflected energy from said auxiliary transmission line substantially equal to the level of reflected energy produced on said main transmission line from said utilization device.

References Cited in the file of this patent UNITED STATES PATENTS Re. 23,131 Webber June 28, 1949 2,188,389 Cork et al. Jan. 30, 1940 2,532,539 Counter et al. Dec. 5, 1950 2,561,212 Lewis July 17, 1951 2,586,993 Riblet Feb. 26, 1952 2,593,120 Dicke Apr. 15, 1952 2,634,331 Honda Apr. 7, 1953 2,636,116 Taylor Apr. 21, 1953 2,644,928 Norton July 7, 1953 

